Removal of oil, dirt, lubrication greases, metal oxides, welding scale etc. is essential prior to the powder coating process as this can cause problems like uneven coating or problematic chemicals mixing in with the powder coat. It can all be removed using a variety of chemical and mechanical methods with the most popular being shot blasting.

In many high end applications, the part is electro-coated following the pre-treatment process, and subsequent to the powder coating application. This has been particularly useful in automotive and other applications requiring high end performance characteristics. This pre-treatment is more expensive and is unnecessary in most of today’s applications.

Another method of preparing the surface prior to coating is known as abrasive blasting or shot blasting. Blast media and blasting abrasives are used to provide surface texturing and preparation, etching, finishing, and degreasing. Shotblasting is a method used to clean, strengthen (peen) or polish metal and is typically done in a blast room. The blast room is a three part system; the containment structure, the abrasive blasting system and the dust collector. Most blast rooms will have a recycling system of some description. This can range from manually reclaiming the abrasive blast using a shovel to reclaiming floor systems that convey the abrasive into a cleaning device before ending back up in the blast pot.

Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles the media and is environmentally friendly. Shot blasting is used in almost every industry, it is a highly efficient preparation for nearly all metal items including aerospace, automotive, construction, foundry, shipbuilding, rail, I-beams, angles, pipes, tubes and large fabricated pieces.

Different powder coating applications usually require methods of preparation such as shot blasting prior to coating. The online consumer market typically offers shot blasting services coupled with their coating services at additional costs.

Shot Blasting Standards

The life of paint, powder and other coatings applied to the surface of steel depends largely on how the surface of the steel has been prepared initially.

It essential that all mill scale, rust and previous coatings have been totally removed prior to painting and coating so that the new coatings are able to adhere to the steel. Coating over anything other than a bare white metal surface will lead to premature failure of the coating - rectification will be expensive as the coatings will have to be removed, the steel surface prepared properly and new coatings applied. If the steel has to be dismantled and removed from site then the cost of rectification will be out of all proportion to the initial cost of the project.

Shot blasting will ensure that the steel is cleaned to bare white metal, SA2.5 / SA3, giving the best possible surface essential for the application of paint, powder and other coatings. The standard surface treatment quality is usually SA 2.5.

What is SA 2.5?

Shotblasting , Shot Blasting Standards & Guidelines

Shotblasting is a means of cleaning steel by blowing an abrasive media against the steel using compressed air, or mechanical means to propel the shot.

Shot Types

Historically, the material used for artificial sandblasting was sand that had been sieved to a uniform size, and hence the term 'sandblasting'. The silica dust produced in the sandblasting process caused silicosis after sustained inhalation of dust. Other materials for sandblasting have been developed to replace sand; for example, steel grit, steel shots, copper slag, glass beads (bead blasting), metal pellets, dry ice, garnet, powdered abrasives of various grades, powdered slag, and even ground coconut shells or corncobs, walnut shells, baking soda have been used for specific applications and produce distinct surface finishes.

Cleaning Steel

For coating applications, there are two main surface contaminants that need to be removed. Mill scale is the grey flakey oxide of iron that’s present on hot rolled steel. It only forms at high temperature in the hot rolling process, so is not present in cold rolled steel. For most coating processes this must be removed, otherwise the coating only adheres to the oxide, not to the steel, and adhesion is reliant on this poor bonding of steel and mill scale.

The other oxide is brown, commonly called rust, and must also be removed. The extent of rust depends on time and conditions of storage, and can represent even millimeters thick deposits. It all must come off.

Standards

ISO 8501-1 is the standard that covers blast cleaning, and it covers blast, hand flame and acid cleaning. The best of the methods is shot blasting and has become the industry standard with various grades as represented in the chart below.